This invention relates to a thin film transistor high voltage electrographic writing head for recording upon a medium. In particular, the writing head comprises thin film elements including stylus electrodes or nibs, dynamic shift register elements or decoder elements, driving buffers, memory or static RAM (Random Access Memory) cells, latches and high voltage drivers.
Amorphous silicon, a-Si, technology has found numerous applications because of its low cost and compatibility with low temperature glass substrates. Circuits are regularly fabricated with linear dimensions in excess of 30 cm. Thin film transistors, TFTs, are widely used as pixel addressing elements in large area active matrix liquid crystal displays, and in printing and scanning bars. Printing systems upon lonography and Electrography have also been demonstrated with a-Si.
An example of a typical electrographic writing head 10 is schematically illustrated in FIG. 8. Such a writing head, manufacturable by thin film fabrication techniques, is fully disclosed in U.S. Pat. No. 4,588,997 to Tuan et al. which is hereby incorporated by reference. An example fabrication technique is also discussed in U.S. Pat .No. 4,998,146 to Hack.
Writing head 10 comprises a linear array of several thousand styli or nibs 12 for generating sequential raster line of information by means of high voltage electrical discharges across a minute air gap to a conductive electrode. In order to drive selected styli in the array a multiplexing scheme is used wherein the charge on each stylus is controlled by a low voltage thin film pass transistor (LVTFT) 14 which selectively charges and discharges the gate of a thin film high voltage transistor 16 for switching the HVTFT. This scheme allows each stylus to maintain its imposed charge, for substantially a line time, between charges and discharges. The drain electrode 18 of HVTFT 16 is connected to high voltage bus 20 (maintained at about 450 voltage relative to ground) via load resistor 22, and its source electrode 24 is connected to ground bus 26. Data signals, from data lines 28, on the order of 20 volts (ON) and 0 voltage (OFF) will be imposed upon the gate electrode of the HVTFT when the address line 30 switches the gate of LVTFT 14 between about 24 volts (ON) and 0 volts (OFF) during "gate time" of about 15 to 25 microseconds, i.e. the time it takes for the gate of the HVTFT to reach its desired potential.
Writing takes place in electrography when the potential difference between the stylus 12 and a biased complementary electrode (not shown) is sufficient to break down the air gap therebetween, in one form of this art, the complementary electrode is biased to a potential of several hundred volts. In the ON state of the HVTFT 16 writing will take place because the stylus will achieve a low potential so that the difference between it and the complementary electrode is high enough to cause air gap breakdown. When the HVTFT is ON, a current path exists from the high voltage bus 20 to ground through the HVTFT, and the large voltage drop across the load resistor 22 will cause the potential on the stylus 12 to approach ground (typically about 10 volts). In the OFF state of the HVTFT no writing will occur because no current path exists from the high voltage bus to ground, there will be no potential drop across the load resistor, and a high potential (of about 450 volts) will be applied to the stylus 12.
There are a variety of advantages to large area technology when it is applied to input or output devices. For many competing technologies some form of magnification is needed to scale up the system, for example laser printing or CCD scanning require optical magnification. Printing and scanning systems built in large area technology contain fewer mechanical and optical parts so that reliability can be higher. For instance, U.S. Pat. No. 4,466,020 to O'Connell describes an integrated imaging bar having both an array of photosensitive elements and an array of associated marking elements. Moreover, with integrated electronic content on the input or output devices, the number of interconnections may be reduced. Therefore, it would be advantageous to integrate more functionality onto an electrographic writing device than what is shown in FIG. 8.
The most striking feature of the drive characteristics of a-Si TFTs is the low output current. These transistors have both a low mobility and a larger threshold voltage (1 V to 2V). As stated above, the mobility is nearly three orders of magnitude below crystal silicon. To partly compensate for the low current drive, higher operating voltages are used. The low voltage transistors can withstand gate to source potentials up to 40 V without failure. However even with the higher drive voltage, the gate select time is tens of microseconds.
Besides the speed, another complication is the threshold voltage shift. This is much faster in a-Si than in crystal silicon. Rises of up to 5 V in an operational lifetime are seen and must be compensated for in the circuit design. Because of the higher threshold voltages and the slow speed of the TFTs, operating voltages are typically 15 V to 25 V. This complicates the input to large area circuits because level shifting buffers must be used. A large number of level shifters can add a significant cost to a system. It is therefore desirable to have as few inputs as possible thus reducing the number of level shifting buffers required. Furthermore, a reduction in input pads on an integrated circuit such as disclosed herein will typically increase reliability and decrease cost.
The device discussed herein is a 400 driver per inch print array. The array has 32 parallel data drivers per segments, resulting in 148 segments; each being controlled by a single select line. Such a large number of inputs can drive up the cost of the array interface significantly, for reasons already discussed. Since the speed requirements of the select lines is low, there is opportunity to reduce the number of inputs even further by moving the select line drive circuitry directly into the a-Si array. One method is to integrate into the array an a-Si serial-in/parallel-out shift register; whereby shifting a single active bit down the register enables each of the segments in turn. Another configuration could be the use of a dynamic shift register as described in cross-referenced application titled "Parallel Multi-Phased a-Si Shift Register for Fast Addressing of an a-Si Array". Use of such a shift register would allow the writing head to operate at a higher speed. Still another configuration could be to use an integrated decoder for selecting segments on the array. Although an a-Si device is described herein, it can be appreciated that the following invention could be made from non-crystalline silicon (e.g. poly-crystalline, micro-crystalline).
In the design illustrated in FIG. 8, the storage of data on the print head is dynamic, thus needing refresh. The necessary refresh adds additional burden to the computer interface since it must receive new data, store the data needed for the refresh, and schedule the transmission of new data between refresh cycles. Furthermore, the device of FIG. 8 also presents data to the high voltage output drivers as the data is being written to the head. Movement of the media is in the process direction and one segments of an entire scanline is written at a time. For high speed wide format drivers (e.g. 36 inch), where three adjacent print heads are printing in parallel, this approach comprises horizontal or scanline (along the printhead) line quality. By having the data buffered in the print head in an intermediate stage, the data could be presented to the high voltage output drivers all at once across the entire scanline after all of the memory is loaded and latched into place. Printing in this manner yields a higher degree of print quality.
Therefore, in light of the above discussion, it would be highly desirable to have a fully integrated writing head which increases functionality while decreasing the number of input pads. Furthermore, such an integrated device could have on board storage capability resulting in improved print quality.